Metal cation s lone-pairs increase octahedral tilting instabilities in halide perovskites

Abstract

Halide perovskites exhibit beneficial opto-electronic properties (e.g. long carrier lifetimes and low defect densities), and their dynamic structural instabilities and anharmonic thermal fluctuations are directly implicated in these properties. In this work, we combine in-depth analysis of Raman spectroscopy and ab initio calculations to uncover the critical roles of Group 14 M²⁺ (M = Pb, Sn, Ge) metal cation s orbital lone pairs in the dynamic instabilities of CsMBr₃ and particularly in governing the octahedral tilting. Previous studies concluded that the lone-pair stereochemical activity primarily leads to the off-centering motion of the metal cation, and the tilting is usually ascribed to ionic size effects. Here, we show that the lone-pair stereochemical activity contributes to strong octahedral tilting instabilities that induce liquid-like behavior in all examined crystals, which underlies the robustness of halide perovskites to charged defects. In addition, the lone-pairs induce a local, molecule-like behavior of the Ge²⁺ with a pyramidal bonding motif in the cubic phase, and they contribute to another phase transition of CsSnBr₃ at 60 K. Our findings elucidate the fundamental origin of anharmonicities in halide perovskites and provide the crucial link between chemical composition and optoelectronic properties, opening opportunities for lead-free and solution-processable photovoltaics.